Well, the very impressive dyke intrusion which has been steadily growing over the last week or so appears to have broken the surface. A 1 km long fissure eruption started early this morning and is ongoing – see this webcam for a good aerial view (although it seems to be getting hit pretty hard with traffic, so the connection can be a bit flaky). EDIT: There’s a second webcam here, which is behaving better, and you can just see the eruption in the distance. The image below is a screencapture with the fissure eruption circled.
A fissure eruption is exactly what it sounds like – an event where eruptive material exits along a linear fissure. Iceland is particularly well known for fissure eruptions, although they can occur anywhere you have dykes being injected; these subterranean magma sheets are what feed these linear features at the surface. They are particularly common in extensional tectonic settings, such as Iceland, which of course sits on top of the North Atlantic spreading axis.
The eruption so far seems to be pretty low energy and output, with steam and tephra being fountained just a few hundred meters into the air (my own estimate from the webcam images). This might be a function of the magma cooling rather rapidly as it progresses through the dyke, resulting in a somewhat more viscous melt, which generates a tephra-rich eruption rather than the effusive fire-fountaining more typical of these types of event. For more information on the magma being erupted, have a look at this contribution from Christina Manning, an expert in Icelandic geochemistry, who specialises on these volcanoes.
EDIT 11am 28/8/14: There’s some great aerial footage, from a light aircraft flying along the length of the fissure here: http://www.ruv.is/frett/fyrstu-flugmyndir-af-eldgosinu. You can clearly see the spatter ramparts that have been built up by the early stages of the eruption, now only venting gases.